Automatic trim system for aircraft



Aug 23, 1965 J. R. YOUNKIN ETAL 3,268,187

AUTOMATIC TRIM SYSTEM FOR AIRCRAFT Filed April 1'7, 1964 2 Sheets-Sheet 1 my. j

J. R. YOUNKIN ETAL.

AUTOMATIC TRIM SYSTEM FOR AIRCRAFT Aug. 23, 1966 Filed April 17, 1964 United States Patent O 3,268,187 AUTQMATC 'IREM SYSTEM FOR AliRCRAFT James R. Younkin, Don H. Mitchell, and Daniel M. Thurman, all of Mineral Wells, Tex., assignors to Mitchell Industries, Inc., Mineral Wells, Tex., a corporation of Texas Filed Apr. 17, 1964, Ser. No. 360,497 9 Claims. (Cl. 244-83) The present invention relates to automatic control systems, and more particularly, but lnot by way of limitation, relates to an automatic pitch trim system for an aircraft or other vehicle having similar control modes.

In normal flight, it is necesasry to adjust or trim the controls of an -airplane in order to maintain the desired flight attitude without continued manual effort on the part of the pilot, or continued operation of an autopilot. In most aircraft, the pitch control is subject tothe greatest movement in order to attain all the necessary flight attiudes. Accordingly, nearly all aircraft are provided with a manually-actuated pitch trim which must be adjusted to large degrees during take-off, climb, approach and landing, and from time to time during normal liight as a result of changes in power settings, changes in the weight of the aircraft due to use vof fuel and shift of passengers, or when changing altitude. The manually-operated trim tab systems require a considerable amount of attention, particularly during the approach and landing when the maximum demand is made upon the pilots attention for other matters.

If the aircraft is not properly trimmed while being controlled by an automatic pilot, the autopilot will be required to continuously exert a corrective force on the control. This, together with changes in the trim requirements during the course of autopilot liight, frequently results in relatively high and occasionally excessive loads on the auto-pilot. In addition to being deleterious to the autopilot, the improper trim of the aircraft can become a hazard because when the autopilot becomes overburdened, the `aircraft becomes unstable, and the pilots first instinct is to quickly de-activate the autopilot. This sometimes results in an abrupt change in the attitude of the aircraft which can Ibe quite hazardous and at best is undesirable.

The pitch control or elevator of most light aircraft is controlled by a pair of tension cables which extend in generally parallel relationship from the cokpit to the elevators -to actuate a bellcrank which is connected to raise and lower the elevators. The manual pitch trim control is usually comprised of a wheel or hand crank which is connected through cables and suitable linkage to a trim tab located on the movable elevator control surface. Operation lof the pitch trim control repositions the trim tab in such a manner as to change the aerodynamic effect of the elevator or pitch control element.

The present invention contemplates an automatic trim system by which tan aircraft can be trimmed merely by exerting the customary pressure above the control wheel or stick, as the case may be, necessary to effect the desired flight attitude and closing a switch to energize the automatic trim system. The force exerted rby the pilot is then sensed and a servo system actuated to move the trim tab in a manner to trim the airplane until no further force is required to maintain the selected flight attitude. Automatic pitch trim systems for accomplishing this purpose have been heretofore available. However, such devices have been relatively complex and by and large have been limited to large and expensive aircraft wherein the automatic trim systems have been incorporated in the aircraft control system at the time of manufacture.

The present invention is concerned with an automatic 3,Z8,l37 Patented August 23, 1966 trim system which is particularly adapted for use in light aircraft of the type which utilize cables for actuating a control s-urface, and which have manually-actuated trim tabs associated with the control surface to change its aerodynamic effect.

In general, the automatic trim system is comprised of first and second electric switch means; means for comparing and detecting an imbalance between first and second forces applied to the control surface, and for closing the first switch means when the first force exceeds the second force, and for closing the second switch means when the second force exceeds the first force; electric servo means for actuating the trim system in either direction; rst circuit means for slowly energizing the servo means in response to closing of the first switch means to actuate the trim system in a manner to reduce the first force and for quickly de-energizing the electric servo means upon opening of the first switch means, and second circuit means fo-r slowly energizing -the electric servo means in response to closing of the second switch means to act-uate the trim system in -a manner to reduce the second force, and for quickly de-energizing the electric servo means upon opening of the second switch means.

In accordance with a more `specic aspect of the invention, the means for comparing and detecting an imbalance between the first and second forces is comprised of a member pivotally supported for pivotal movement about an axis for closing 4the first switch means and opening the second switch means when pivoted in a first direction about the axis 'and for closing the second switch means and opening the tirst switch means when pivoted in the second direction about the axis, first means for engaging and deflecting a lirst control cable such that tension in the cable will tend to pivot the member about the axis in the first direction with a force proportional to the tension in the cable and second means for engaging and deflecting the second control cable such that tension in the cable will tend to pivot the member in the second direction about the axis with a force proportional to the tension in the second cable.

The present invention also contemplates a novel amplifier circuit means for energizing the servo means in response to closing one of the switch means as a result of an imbalance between the forces applied to the control surface comprising a transistor the emitter of which is connected through a relatively small resistor to a power voltage, the collector of which is connected to ground, the base of which is connected through a capacitor to the power volt-age and through a relatively large resistor to the respective switch means, and a power circuit connected to the electrical servo system for energizing the servo system responsive to conductance of the transistor.

Therefore, an important object of the present invention is to provide an automatic pitch trim system of the type described which may be installed in most light aircraft without appreciable al-teration of the existing control system.

Still .another object of the present invention is to provide a system of the type described having increased sensitivity.

Yet another object of the present invention is to provide a system of the type described which is highly reliable.

A still further object of the invention is to provide a system of the type described which may be used in conjunction with any conventional type of autopilot so as to continuously maintain the aircraft in proper trim and thereby reduce the load on the autopilot and eliminate violent reactions as a result of de-energization of the autopilot.

Another object of this invention is to provide a highly simplified sensing device for comparing and detecting an imbalance in the forces applied to a control surface of an aircraft or -the like.

Still another object of the invention is to provide an improved servo circuit means operative in response to the alternate closing and opening of two switches .to drive a servo-motor or the like in such a manner as to alleviate the command situation causing one of the switches to close.

Yet anotherobject of the present invention is to provide a simplified servo system for automatically maintaining a predetermined condition.

Many additional objects and advantages will be evi-dent 'to those skilled in the art from the following detailed description and drawings, wherein:

FIGURE l is a schematic perspective view of a system constructed in accordance with the present invention;

FIGURE 2 is a side View of the force-imbalance sensing devi-ce of the system of FIGURE l;

FIGURE 3 is an end view of the force-imbalance sensing vdevice of FIGURE 2;

FIGURE 4 is a top view of the force-imbalance sensing device of FIGURE 2;

FIGURE 5 is a partial sectional view of the forceimbalance sensing device of FIGURE 2 `taken generally 0n lines 5 5 Aof FIGURE 2; an-d,

FIGURE 6 is a schematic circuit diagram of the automatic ltrim system constructed in accordance with the present invention.

Referring now to the drawings, and particularly to FIG- URE 1, the pitch of an aircraft 10 is controlled by a manually-operated control wheel 12 which actuates a pitch control sur-face 16 or elevator through la conventional linkage system including a pair of cables 14. For example, when the control wheel 12 is pushed forward by the pilot, the trailing edge of the control surface 16 will be lowered and the aircraft moved into a relative nose-low attitude. The forces tending to neutralize the control surface 16 result in an imbalance between the tensions in the cables 14, with a greater tension being present in the cable which moves the trailing edge of the control surface downwardly. Conversely, when the back 'pressure is exerted on the con-trol wheel 12, the trailing edge of the control surface is moved upwardly to cause the nose of the aircraft to rise and the tension in the other of the cables becomes greater.

In order to maintain the aircraft in either a nose-low or nose-high attitude, it is necessary to continually exert pressure on the control wheel or the aircraft will tend to return to the flight -attitude for which it is trimmed. However, the aircraft can be trimmed to any set attitude by a hand crank 20, for example, which actuates a trim tab surface 22 through cables 24 to change the aerodynamic effect of the control surface 16 and thereby change the stabilized iiight attitude of the aircraft.

An automatic pilot 26 may be used to maintain a preselected iiight attitude with regard to the pitch axis, or a pre-selected altitude. The autopilot 26 is coupled to the control system and also actuates the control surface 16 through the cables 14. When using the au-topilot, it heretofore has been necessary to first manually trim the aircraft using the hand crank to the desired flight attitude. Then the autopilot 26 controls the pitch of the aircraft through the manual control cables 14 in much the same manner as the pilot and produces tensions in the cables 14 whenever pressures are applied to the control system to maintain the pre-selected flight attitude.

The present invention is comprised of a system for detecting an imbalance -between the tensions in the cables 14 and for automatically adjusting the trim of the aircraft through the trim tab control cables 24 so as to eliminate the imbalance. This alleviates the necessity of manually trimming the aircraft with the hand crank 20 when the aircraft attitude is controlled either manually or by the autopilot 26, as will hereafter be described in greater detail.

The imbalance in tension of the cables 14 is detected by a sensing device indi-cated generally by the reference numeral 30 and illustrated in FIGURES 2-5. The sensor 30 is comprised of a support plate 32 which may be secured to the air frame of the aircraft 10 by any suitable means (not shown). A journal block 34 is connected to the underside of the support plate 32 and a member 36 is pivotally connected to the journal block by a suitable pivot pin 38 such that the member 36 will pivot about `an axis extending generally along the axis of the pin. An axle 42 is connected to the member 36 generally 'at right angles to the pivotal axis formed by the pin 3S. A pair of pulleys 40a and 40h are journaled on the axle 42 at opposite sides of and equidistant from the pivot pin 38. The pulley 40a engages and deilects the cable 14a, and the pulley 40]; engages and deflects the cable 14h. Pillow blocks 46a and 46b are connected to finger portions 44a tand 4411, respectively, of the support plate 32. The ends of an axle 48 are secured in the pillow blocks 46a and 4619 and the axle 48 extends generally parallel to the axle 42. A second pair of larger pulleys 50a and 50b are journaled on the axle 48 and engage and deflect the cables 14a and 1412, respectively, in directions opposite to the directions in which the respective cables are deected by the respective pulleys 40a and 40b. The pulleys Stia and Stib serve to amplify the effect of the tension in the cables 14a and 14b on the pulleys 46a and 46h as will presently be described. The relatively thin finger portions 44a and 44b serve as a rather stiff spring means to provide some degree of resiliency to the force applied to counterdeflect the cables 14a and 14b. From the description of the pulley larrangements, it will be evident that the tension in the cables 14a and 14b tend to pivot the member 36 in opposite directions aboutthe axis formed by the pivot pin 38. W'hen the tensions in the cables are equal, the mem- 'ber will be centered. However, any difference between the tensions will cause pivotal movement of the member 36 about the pin 38.

A pair of switch means which are `alternately closed and opened as a result of pivotal movement of the member 36 can best be seen in FIGURE 5. The left-hand end of the member 36, when referring t-o FIGURE 2, is provided with a portion that extends above the pivot pin 38 and forms a switch blade 52. A pair of insulating ,plates 54a and 54b `are connected to the journal block 34 and extend to points on opposite sides of the switch blade portion 52. A pair `of electrical contacts 56a and 56h are positioned between the insulated plates 54a and 5411 for alternate engagement with the switch blade portion 52 as the member 36 is pivoted about the pin 38. The relative positions of the contacts 56a and 56h may be adjusted by screws 58a and 5819 to adjust the device and increase its sensitivity. Thus if the tension in the cable 14a exceeds the tension in the cable 14h, the member 36 will be pivoted a first -or clockwise direction when referring to FIGURE 3 such that the switch blade portion 52 will close against the Contact 56a, and contact 56h will be opened. Conversely, if the tension in cable 14b exceeds the tension in cable 14a, the member 36 will be pivoted in the counterclockwise or second direction when referring to FIGURE 3 such that the switch blade 52 will close against the contact 56b, and the contact 56a will be open. The member 36, and therefore the switch blade 52, may be connected to ground by a cable 60. The contacts 56a and 56h may be connected to con` trol circuits by any suitable leads (not illustrated). The control circuits may conveniently be located in the housing 62, and will now be described.

Referring to the circuit diagram of FIGURE 6, a positive voltage terminal 64 is connected by a circuit breaker 66 to a manually-actuated switch 68. The switch 68 is preferably comprised of a manually-operated button 70 positioned on the control wheel of the aircraft so that the pilot can close the switch without removing his hand nected to the base of a transistor 11061.

from the control wheel. The switch 68 has a irst contact 72 which is connected to ground and a second contact 74 which is connected to the circuit breaker 66. The switch blade 76 is spring-biased into engagement with the contact 72 and is manually closed against the contact 74 by pressing the button 70. The switch blade 76 is connected by lead 78 to the contact 79 of a second switch Si). The switch 80 has a switch blade 82 which is spring-biased against the contact 79 and is moved by a cam against a contact 84 whenever the automatic pilot is actuated to exert a force on the pitch control surface 16. The switch blade 82 is connected by lead 86 and jumper lead 88 to terminal 8,9 of a trim tab servo system indicated generally by the reference numeral 90.

The trim tab lservo system 90 has a reversible motor 91 having an armature winding 92 and split field windings 94a and 94h. The armature winding 92 is connected to terminal 89. The solenoid 96 of a magnetic clutch (not illustrated) is also connected to the positive terminal S9 and to ground. The trim tab servo system 90 is coupled through the magnetic clutch to drive the trim tab cables 24 in alternate directions so as to trim the aircraft as will hereafter be described. The positive terminal S9 is connected through lead 98 to a dual amplier circuit means indicated generally by the refe-rence numeral 10i). Each of the amplifier circuits is identical and, for simplicity, corresponding components are therefore designated by the same reference numerals followed by the subscripts a or b as will hereafter be evident.

The emitter of a transistor 102a is connected through a relatively small resistor 104a to the llead 98 which provides -a source 4of positive voltage. The collector of the transistor 10241 is connected to the contact 56a of the `sensing device 30. The base of the transistor 102a is connected by a capacitor 106a to the positive voltage lead 93 and through a relatively large resistor lllSa4 to the contact 56a `of the sensing device 30. The switch blade portion 52 of the sensing element is connected to ground. The emitter =of the transistor 10211 is also con- The emitter of the transistor 110:2 is connected by lead 112a to the-field winding 94a of the motor 91 and the collector is connected to ground. The resistor 10451 may be ion the order of 470 ohms while the resistor 108:1 may be on the order of 3.3 kilo ohms. The other half of the circuit .is of identical construction but is connected to the contact 56h of the sensing device 30 and to the eld winding 94b of the motor 91.

Operation Assume now that the automatic pitch trim system is to be manually operated in response to manual actuation of the control wheel. The automatic pilot would be deactivated and the switch blade S2 of the switch 80 would be closed against the upper contact 79. The manuallyoperated control wheel switch 68 would be :spring-biased against the contact 72. The high voltage terminal 89 is then connected to ground through leads 88 and 86, contact 79, switch blade 82, lead 7S, switch blade 76 and contact 72 so that the motor 91 is shorted to ground and cannot possibly be operated. Assume for purposes of illustration that the aircraft has just taken olf and it is desired to trim the aircraft in a climb attitude. The pilot would then exert back pressure on the control wheel and press the button 70. The switch blade 76 would be closed against the contact 74 and the positive terminal 89 energized through the contact '79 of the switch 8i). At the same time, assuming that the cable 14a actuates the control surface in such a manner as to cause the aircraft to assume a climbing attitude, the switch blade portion 52 would be closed against the contact 56a as a result of the imbalance between the tension -in the cable 14a and lthe cable 1421 which would pivot the member 36 in the clockwise direction, when referring to- FIGURE 3, about the pivot pin 38.

When both the manually-operated switch blade 76 is closed against the contact 74 and the switch blade portion 52 is closed against the contact 56a, the base of the transistor 10211 will be connected to ground through the resistor ltla so as to slowly charge the capacitor 10651. Since the resistor 108:1 is quite large, a significant time delay will occur before the voltage at the base of the transistor 1tl2a is reduced sufliciently with respect to ground to begin to turn the transistor on As the impedance of the transistor 102a is reduced, the base of the power transistor 11011 becomes more negative and the impedance of the power transistor 11001 is reduced substantially linearly. Current then passes through the armature winding 92 of the series-Wound motor 91 and through the field winding 94a to drive the motor. As the switch blade 76 is closed against the contact 74, the magnetic clutch solenoid 96 is al-so energized to couple the trim tab servo system 9i) to the trim tab cables 24. The motor 91 then drives the trim cables 24 in the direction required to trim the aircraft in the desired climbing attitude. Actuation of the trim tab cables continues until such time as the tension in the elevator control cables 14 is again equalized due to the adjusted trim of the aircraft. Then the switch blade portion 52 opens from the contact 56a. When the contact 56a opens, the capacit-or 106g very quickly discharges through the loop comprised of the relatively small resistor 104er and the emitter and base of the transistor 102:1 which then merely functions as a diode because the operating potential has been removed from the collector. 'Ihe rapid discharge of the capacitor 106e: results in a rapid rise in the impedance of the transistor 102a and consequently in the transistor 11th: to very quickly de-activate the servomotor 91 and accurately stop the trim tab control surface at the desired position.

Operation of the automatic trim system to automatically trim the airplane in the nose-down or descending attitude is accomplished in precisely the same manner except that the tension in cable 14b overbalances the tension in the cable 14a so as to pivot the member 36 and close the switch blade portion 52 against the contact 56h. The transistor 102]) is then relatively slowly turned on as the capacitor 106b is charged to slowly turn the power transistor 110b on Current then passes through the armature winding 92 and field winding 94h to drive the servomotor 91 in the opposite direction to that previously described and move the trim tab control surface in such a manner as to equalize the tensions in the control cables 14a and 14b.

When the automatic pitch trim system is operated in conjunction with an autopilot, the switch blade 76 of the pilots control switch 68 is springabiased against the ground contact 72. Then when the autopilot is activated, but not engaged, the switch blade 82 of the switch 80 will be closed against the contact 79 as a safety measure to insure against accidental operation of the servomotor 91. However, when the automatic pilot exerts a force on the control system, a cam moves the switch blade 82 against the contact 84 so as to energize servo system 90 and circuit means 100. The automatic pitch trim system will then function in the manner previously described any time that the autopilot causes the tension of one of the cables 14a or 14h to exceed the tension in the other in order to maintain the preselected flight attitude or altitude.

It is important to note that in order to have a highly sensitive and accurate system, the device 30 must compare the tension in the cable 14a with the tension in the cable 14b and detect any minute difference. This can be accomplished only if the contacts 56a and 56h are adjusted extremely close to the switch blade portion 52 so as to reduce the travel required of the switch blade portion before it closes against the contacts to a minimum. Thus it will be appreciated that when the tensions in the two cables are substantially equal, a certain amount of rapid opening and closing of the contacts, or

' acute in turbulent air.

' up time-constant.

This condition is particularly Therefore, it is important that some means be provided to prevent false actuation of the system. This is accomplished by a very rapid response time when going from the on state to the off state as compared to going from the off state to the on state. More specifically, the circuit means 100 has a much more rapid decay time-constant than build- The relatively large resistor 108b controls the rate at which the capacitor lifla is charged and the capacitor must be charged `to turn the transistor 132g chattering will result.

' on The relatively small resistor 10aa controls the rate at which the capacitor 106:1 discharges each time that the contact 56a is opened. Thus if the contact 56a is closed a number of times in quick succession, the capacitor 106g cannot be charged to a sufficient extent to turn the transistor 10Za on because each time the contact 56a opens, the charge on the capacitor lflda is quickly dissipated through the emitter base of the transistor 1.0M which functions as a diode when the collector circuit is opened. Therefore unless the contact 56a is closed for a sufficient period of time for the capacitor 106a to be charged through the relatively large resistor lifla, the system will not be activated.

It will be appreciated that without this rapid discharge or reset feature, it would be possible to charge both capacitors 106e and 10615 to such an extent as to turn both transistors llfla and llfib on if the switch blade portion 52 were to chatter between the contacts 56a 'and 55h. This would energize both field windings 94a and 94h at the same time and probably damage the motor 91. It will also be appreciated that the circuit illustrated in FIGURE 6 is highly stable in operation. For example, when the contact 56a is open, the emitter and base of the transistor lima are at essentially the same voltage so that the transistor is biased off Similarly, the emitter and base of the transistor llfla are at substantially the same voltage so that the transistor is off Conversely, when the contact 56a is closed, the base of the transistor 1f2a is connected through the resistor lflSa to ground so that the impedance of the transistor 10251 is reduced to a low value, and the base of the transistor llfla is then reduced to a low voltage level to insure a positive switching action.

From the above detailed description of a preferred embodiment of the invention, it will be evident that a simple l and economical automatic trim system which can be used in combination with the standard control system of most light aircraft has been described. The system is reliable and safe and is highly sensitive. A novel means for comparing the tension in two .generally parallel cables and detecting any imbalance has been described which is very simple, reliable and economical to manufacture. A novel circuit means having a long build-up timeconstant and a short decay time-constant has been de- 'scribed which is reliable and positive-acting, yet is very sensitive and will not be activated by false signals. The system may be used when the airplane is being controlled manually or by autopilot.

Having thus described a preferred embodiment of the present invention in detail, it is to be understood that various changes, substitutions and alterations can be made in the component parts and in the combination thereof without departing from the spirit and scope of the invention as defined by theV appended claims.

What yis claimed is:

1. An automatic trim system for an aircraft or the like having a control element actuated in opposite directions by first and second forces and trim means associated with the control element for changing the effect `of the control element in such a manner as to balance the forces required to maintain a desired fiight attitude, comprising:

.f5 the first switch means when the first force exceeds the second force and for closing the second switch 'means when the second force exceeds the first force, electric servo means for actua-ting the trim system, first circuit means for energizing -the servo means in response to closing of the first switch means to actuate the trim system in a manner to reduce the first force to the level of the second force, the first cir cuit means having a relatively slow time-constant when changing from off to on and a relatively fast time-constant when changing from on to off, and second circuit means for energizing the servo means in response to closing of the second switch means to actuate the trim system in a manner to reduce the second force to the level of the first force, the second circuit means having a relatively slow time-constant when changing from off to on and a relatively fast time-constant when changing from on to offf whereby the system will have a high sensitivity and yet the servo means will not be energized by rapid opening and closing of the switches.

2. An vautomatic trim system as defined in claim 1 vwherein the means for comparing and detecting an imbalance between the first and second forces comprises:

a member pivotally supported for pivotal movement about an axis for closing the first switch means and opening the second switch means when pivoted in a first direction about the axis, and for closing the second switch means and opening the first switch means when pivoted in the second direction about the axis,

means for pivotally moving the member in a first direction about the axis with a force proportional to the first force, and

means for pivotally moving the member in a second direction about the `axis with a force proportional to the second force,

whereby the member will be pivoted by an imbalance between the forces and close the switch means corresponding to the greater force.

3. An automatic trim system as defined in claim 1 wherein first and second forces are applied to the con- `tro1 element by first and second parallel cables, respectively, and the means for detecting an imbalance between the first and second forces comprises:

a mem-ber pivotally supported for pivotal movement about an axis for closing the first switch means and for opening the second switch means when pivoted in a first direction about the axis, and for closing the second switch means and opening the first switch means when pivoted in the second direction about the axis,

first means connected to the member for engaging and deflecting the first cable such that tension in the first cable will tend to pivot the member in the first direction, and

second means connected to the member for engaging and defiecting the second cable such that tension in the second cable will tend to pivot the member in the second direction,

whereby the member will be pivoted by an imbalance in the tension of the cables in such a manner as to close the switch means corresponding to the cable lhaving the greatest tension.

4. An automatic trim system as defined in claim 3 further characterized by:

third means spaced from the first means for deecting the first cable in a direction opposite to the direction in which the first means deflects the cable, and

fourth means spaced from the second means for deflecting the second cable in Ia direction opposite to the direction in which the second means deflects the cable,

whereby the force exerted on the first and second means by tension in the respective cables will be amplified and the sensitivity of the system increased.

5. An automatic trim system as defined in claim 4 wherein:

the first and second means are pulleys journaled on the member,

the member is pivotally connected to a support, and

-the third and fourth means are pulleys journaled on the support.

6. An automatic trim system as delined in claim 1 wherein the first and second circuit means are each cornprised of:

control circuit means including a capacitor, a charging circuit for the capacitor having a relatively high impedance, and the corresponding switch means, such that when the corresponding switch means is closed the capacitor will be relatively slowly charged, and a discharging circuit having a relatively low irnpedance for relatively rapidly discharging the capacitor when the corresponding switch means is opened, and

power circuit means including power switching means for energizing the electric servo means in the corresponding direction when turned on, the power switching means being connected to the capacitor and turned on when the capacitor is charged and oft when the capacitor is discharged.

7. An automatic trim system as defined in claim 1 wherein the first and second circuit means are each comprised of a control circuit comprising a transistor, the emitter of which is connected through a relatively small resistor to a power voltage, the collector of which is connected to ground, andthe base of which is connected through a capacitor to the power voltage and through a relatively large resistor to the respective switch means, and

a power circuit connected to the electric servo means for energizing the servo means with a current proportional to the conductance of the transistor,

whereby when the respective switch means is closed, the capacitor will -be charged relatively slowly and the transistor slowly turned on, and when the switch means is opened, the capacitor will quickly discharge through the loop comprised of the small resistor and the emitter and base of the transistor to prevent energization of the motor during periods of rapid opening and closing of the switch.

8. An automatic pitch trim system for an aircraft having a control element actuated in opposite directions by a system including a hand control and first rand second cables, and trim means associated with the control element for changing the effect of `the control element in such a manner as to balance the tension required in the cables to maintain a desired ight attitude, said pitch trim system comprising:

electric servo means for actuating the 4trim means in rst and second directions having a power =voltage terminal and first and second ground terminals,

tension sensing yand circuit means for comparing the tension in the first and second cables and for driving the electric servo means in the first direction when the tension in the first cable exceeds the tension in the second cable by connecting the first ground terminal to ground and for driving the electro servo means in the second direction when the tension in the second cable exceeds the first by connecting the second ground terminal to ground,

a source of power voltage, and

switch means for alternatively connecting the power Voltage terminal tothe source of power or to ground, the switch means being normally biased to connect the power terminal to ground and manually movable to connect the power terminal to the source of power.

9. An automatic pitch trim system for an aircraft having a control element actuated in opposite directions by a system including a hand control and first and second cables, trim means associ-ated with the control element for changing the effect of the control element in such a manner as to 4balance the tension required in the cables to maintain a desired flight attitude, and an automatic pilot for Aautomatically controlling the pitch of the aircraft when energized, said pitch trim system comprising:

electric servo means for actuating the trim means in first and second directions having a power voltage terminal and first and second ground terminals, tension sensing and circuit means for comparing the tension in the first and second cables and for driving the electric servo means in the first direction when the tension in the first cable exceeds the ttension in the second cable by connecting the first ground terminal to ground and for driving the electric servo means in the second direction when the tension in the second cable exceeds the iirst by connecting the second ground terminal to ground,

a source of power voltage,

first switch means having rst and second contacts and a first switch Iblade normally biased against the first contact and manually movable into contact with the second terminal, second switch means having third and fourth contacts and a second switch blade normally biased against the third contact and moved by activation of the automatic pilot against the fourth contact,

the second switch blade being connected to the power voltage terminal of servo means, the third contact being connected to the first switch blade, and the first contact being connected to ground, and

the second and fourth contacts being connected to the source of high voltage,

whereby when the first and second switch blades are in the normally biased positions, the power voltage terminal will be connected to ground to prevent accidental energization of the servo means, and when either the first or second switch blades are moved against either the second or fourth contacts, respectively, the power voltage terminal will be connected to the source of power voltage to energize the trim system.

References Cited by the Examiner UNITED STATES PATENTS 2,623,717 12/1952 Price 244-82 2,626,114 1/ 1953 Alderson 244-77 2,723,089 11/1955 Schuck et al 244--77 2,753,504 7/1956 Pell S18-297 2,843,344 7/1958 Gibb 244-82 2,923,872 2/1960 Herzog S18-297 2,938,972 5/1960 Bryson 20G-61.8 3,092,696 6/ 1963 Wallace ZOO-61.8

MILTON BUCHLER, Primary Examiner.

ANDREW H. FARRELL, Examiner. 

1. AN AUTOMATIC TRIM SYSTEM FOR AN AIRCRAFT OR THE LIKE HAVING A CONTROL ELEMENT ACTUATED IN OPPOSITE DIRECTIONS BY FIRST AND SECOND FORCES AND TRIM MEANS ASSOCIATED WITH THE CONTROL ELEMENT FOR CHANGING THE EFFECT OF THE CONTROL ELEMENT IN SUCH A MANNER AS TO BALANCE THE FORCES REQUIRED TO MAINTAIN A DESIRED FLIGHT ATTITUDE, COMPRISING: FIRST AND SECOND ELECTRICAL SWITCH MEANS, MEANS FOR COMPARING AND DETECTING AN IMBALANCE BETWEEN THE FIRST AND SECOND FORCES AND FOR CLOSING THE FIRST SWITCH MEANS WHEN THE FIRST FORCE EXCEEDS THE SECOND FORCE AND FOR CLOSING THE SECOND SWITCH MEANS WHEN THE SECOND FORCE EXCEEDS THE FIRST FORCE, ELECTRIC SERVO MEANS FOR ACTUATING THE TRIM SYSTEM, FIRST CIRCUIT MEANS FOR ENERGIZING THE SERVO MEANS IN RESPONSE TO CLOSING OF THE FIRST SWITCH MEANS TO ACTUATE THE TRIM SYSTEM IN A MANNER TO REDUCE THE FIRST FORCE TO THE LEVEL OF THE SECOND FORCE, THE FIRST CIRCUIT MEANS HAVING A RELATIVELY SLOW TIME-CONSTANT WHEN CHANGING FROM "OFF" TO "ON" AND A RELATIVELY FAST TIME-CONSTANT WHEN CHANGING FROM "ON" TO "OFF," AND SECOND CIRCUIT MEANS FOR ENERGIZING THE SERVO MEANS IN RESPONSE TO CLOSING OF THE SECOND SWITCH MEANS TO ACTUATE THE TRIM SYSTEM IN A MANNER TO REDUCE THE SECOND FORCE TO THE LEVEL OF THE FIRST FORCE, THE SECOND CIRCUIT HAVING A RELATIVELY SLOW TIME-CONSTANT WHEN CHANGING FROM "OFF" TO "ON" AND A RELATIVELY FAST TIME-CONSTANT WHEN CHANGING FROM "ON" TO "OFF," WHEREBY THE SYSTEM WILL HAVE A HIGH SENSITIVITY AND YET THE SERVO MEANS WILL NOT BE ENERGIZED BY RAPID OPENING AND CLOSING OF THE SWITCHES. 